Exhaust system for dual fuel engines

ABSTRACT

An exhaust system for a dual-fuel engine includes an exhaust treatment component in an exhaust passageway. The exhaust treatment component is configured to treat exhaust from the combustion of a second fuel and not from combustion of a first fuel. A thermal enhancement device is in communication with the exhaust passageway and positioned upstream from the exhaust treatment component. A controller activates and deactivates the thermal enhancement device based on switching from the first fuel to the second fuel, wherein the first fuel has a higher sulfur content than the second fuel. The thermal enhancement device increases the temperature of an exhaust to combust a residual amount of the first fuel present in the exhaust passageway during the switch between the first fuel and the second fuel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/804,027 filed on Mar. 14, 2013. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to engine exhaust systems for dual-fuelengines.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Engines that operate on dual fuels are common in the marine industrywhere various emission regulations may be in effect in coastal areas,but not in effect while out to sea at a predetermined distance fromland. For example, the engine on a marine vessel may operate using alower-sulfur-containing fuel while close to shore, and operate using ahigh-sulfur-containing fuel while out at sea.

Combustion of sulfur-containing fuels produces exhaust including sulfur.The sulfur in the exhaust can increase production of sulfur oxides(SO_(X)) that may be detrimental to the environment. Further, the sulfurin the exhaust can react with exhaust after-treatment reagents toproduce by-products such as ammonia bi-sulfate. After prolonged periodsof exposure to ammonia bi-sulfates, catalyst-coated substrates of anexhaust after-treatment system can become plugged, which reduces theefficacy of the exhaust after-treatment system.

Some fuels are more apt to producing ammonia bi-sulfates, includingfuels having a higher sulfur content. To address these concerns, fuelsuppliers have developed lower sulfur content fuels. Fuels having lowersulfur content, however, are more expensive due to the increased costsin production of the fuels. In view of these fuel costs, the above-noteddual fuel engines have been developed. During a fuel switch, however, itis not uncommon for residual fuel to remain in the exhaust system. Ifthe fuel in the exhaust system has a higher sulfur content, the residualfuel can produce greater amounts of ammonia bi-sulfate that can, overprolonged periods, plug the catalyst-coated substrates of the exhaustafter-treatment system.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

The present disclosure provides an exhaust system for a dual-fuel enginethat is provided with a first fuel and a second fuel. The exhaust systemincludes an exhaust passageway, with an exhaust treatment componentprovided in the exhaust passageway. A thermal enhancement devicecommunicates with the exhaust passageway and is located upstream fromthe exhaust treatment component, wherein the thermal enhancement deviceis operable to raise a temperature of an exhaust located in the exhaustpassageway during a switch between the first fuel and the second fuelthat is provided to the dual-fuel engine. The exhaust treatment systemcan also include a by-pass pipe in communication with the exhaustpassageway that by-passes the exhaust treatment component wherein,during combustion of the first fuel by the dual-fuel engine, the by-passpipe is open. During combustion of the second fuel by the dual-fuelengine, the by-pass pipe is closed.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic representation of an exhaust system according tothe present disclosure;

FIG. 2 is a flow-chart schematically illustrating a method according toa principle of the present disclosure;

FIG. 3 is a flow-chart schematically illustrating another methodaccording to a principle of the present disclosure; and

FIG. 4 is a flow-chart schematically illustrating another methodaccording to a principle of the present disclosure.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 1 schematically illustrates an exhaust system 10 according to thepresent disclosure. Exhaust system 10 can include at least an engine 12in communication with a pair of fuel source 13 a and 13 b that, oncecombusted, will produce exhaust gases that are discharged into anexhaust passage 14 having an exhaust after-treatment system 16. Fuelsources 13 a and 13 b contain different fuels. For example, fuel source13 a can include a low sulfur diesel fuel (LSF), while fuel source 13 bcan include an ultra-low-sulfur diesel fuel (ULSF). Other exemplaryfuels that can be used include marine gas oil (MOO), marine diesel oil(MDO), intermediate fuel oil (IFO), heavy fuel oil (HFO), combinationsof natural gas and diesel fuel, or blends of natural gas with hydrogen.Further, it should be understood that any combination of theabove-mention fuels can be stored in fuel sources 13 a and 13 b.

Downstream from engine 12 can be disposed an exhaust treatment component18, which can include a diesel oxidation catalyst (DOC), acatalyst-coated diesel particulate filter (DPF) component or, asillustrated, a selective catalytic reduction (SCR) component 20.Although an SCR component 20 is illustrated, it should be understoodthat SCR component 20 can also include therein a DOC or a DPF. Further,SCR component 20 can be an SCR catalyst-coated DPF or an SCRcatalyst-coated flow-through filter (FTF).

Exhaust after-treatment system 16 can further include components such asa thermal enhancement device or burner 17 to increase a temperature ofthe exhaust gases passing through exhaust passage 14. Increasing thetemperature of the exhaust gas is favorable to achieve light-off of thecatalyst in the exhaust treatment component 18 in cold-weatherconditions and upon start-up of engine 12, as well as initiateregeneration of the exhaust treatment component 18 when the exhausttreatment component 18 is a DPF.

To assist in reduction of the emissions produced by engine 12, exhaustafter-treatment system 16 can include an injector or dosing module 22for periodically dosing an exhaust treatment fluid into the exhauststream. As illustrated in FIG. 1, dosing module 22 can be locatedupstream of exhaust treatment component 18, and is operable to inject anexhaust treatment fluid into the exhaust stream. In this regard, dosingmodule 22 is in fluid communication with a reagent tank 24 and a pump 26by way of inlet line 28 to dose an exhaust treatment fluid such asdiesel fuel, urea, or gaseous ammonia into the exhaust passage 14upstream of exhaust treatment component 18. Tank 24 may store liquidexhaust treatment fluids, or may store solid or gaseous ammonia. Othermaterials that can be used to enhance exhaust treatment in combinationwith urea can be ethanol or hydrogen that may be stored in a separatetank (not shown).

Dosing module 22 can also be in communication with reagent tank 24 viareturn line 30. Return line 30 allows for any exhaust treatment fluidnot dosed into the exhaust stream to be returned to reagent tank 24.Flow of the exhaust treatment fluid through inlet line 28, dosing module22, and return line 30 also assists in cooling dosing module 22 so thatdosing module 22 does not overheat. Although not illustrated in thedrawings, dosing module 22 can be configured to include a cooling jacketthat passes a coolant around dosing module 22 to cool it.

The amount of exhaust treatment fluid required to effectively treat theexhaust stream may vary with load, engine speed, exhaust gastemperature, exhaust gas flow, engine fuel injection timing, desiredNO_(x) reduction, barometric pressure, relative humidity, EGR rate andengine coolant temperature. A NO_(x) sensor or meter 32 may bepositioned downstream from SCR 20. NO_(x) sensor 32 is operable tooutput a signal indicative of the exhaust NO_(x) content to an enginecontrol unit (ECU) 34. All or some of the engine operating parametersmay be supplied from ECU 34 via the engine/vehicle databus to an exhaustsystem controller 36. The exhaust system controller 36 could also beincluded as part of the ECU 34. Exhaust gas temperature, exhaust gasflow and exhaust back pressure and other vehicle operating parametersmay be measured by respective sensors, as indicated in FIG. 1.

The amount of exhaust treatment fluid required to effectively treat theexhaust stream can also be dependent on the size of the engine 12. Inthis regard, large-scale diesel engines used in locomotives, marineapplications, and stationary applications can have exhaust flow ratesthat exceed the capacity of a single dosing module 22. Accordingly,although only a single dosing module 22 is illustrated for urea dosing,it should be understood that multiple dosing modules 22 for ureainjection are contemplated by the present disclosure.

During operation of engine 12, as noted above, the type of fuel providedto engine 12 can be switched between different fuel sources 13 a and 13b. In the case where fuel sources 13 a and 13 b carry fuels withdifferent sulfur contents, respectively, it should be understood thatwhen engine 12 is using the fuel with a higher sulfur content,after-treatment system 16 is not necessarily being utilized. That is,when engine 12 is a marine application where the vessel is located apredetermined distance from shore, emission regulations may not requireuse of after-treatment system 16. Accordingly, any exhaust produced byengine 12 while using a high-sulfur-content fuel (or any type of fuel)may be expelled into the atmosphere without passing throughafter-treatment system 16. To expel exhaust directly into the atmospherebefore reaching after-treatment system 16, exhaust system 10 may includean after-treatment by-pass pipe 40. A valve 44 may be positioned at aninlet of by-pass pipe 40 to allow exhaust gas to flow through by-passpipe 40 or through after-treatment system 16. Valve 44 is communicationswith controller 36 or ECU 34. If engine 12 is operating on ahigh-sulfur-content fuel, controller 36 or ECU 34 can instruct valve 44to open by-pass pipe 40 and close exhaust passage 14 downstream of valve44 to allow exhaust to escape into the atmosphere without passingthrough after-treatment system 16. Similarly, if engine 12 is beingoperated in an area where emission regulations require exhaustafter-treatment, controller 36 or ECU 34 can instruct valve 44 to closeby-pass pipe 40 to allow exhaust to pass through after-treatment system16.

Exhaust produced through combustion of fuels that contain sulfur can bemore apt to produce by-products that can plug SCR 20. For example, whenexhaust produced through combustion of sulfur-containing fuel passesthrough exhaust after-treatment system 16 and urea is the reagent beingdosed into the exhaust stream by injector 22, ammonium bisulfate[NH₄]⁺[HSO₄]⁻ can be produced, which after long periods of exposure toSCR 20 can plug SCR 20. Although any sulfur-containing fuel can produceammonium bisulfate, fuels having a lower sulfur content are less apt toproduce ammonium bisulfate. Accordingly, it is preferable that exhaustproduced through combustion of higher-sulfur-containing fuels by engine12 is not passed through after-treatment system 16, but rather expelledthrough by-pass pipe 40 before passing through SCR 20.

In the event of a fuel switch between a high-sulfur-containing fuel anda lower-sulfur-containing fuel during operation of engine 12, however,it can be common for un-combusted fuel droplets of thehigh-sulfur-containing fuel to continue into the exhaust stream duringthe fuel switch. The residual fuel in the exhaust can react with theurea to produce ammonium bisulfate, which over prolonged periods canplug SCR 20. To prevent, or at least substantially minimize residualhigh-sulfur-containing fuel present in the exhaust stream from formingammonium bisulfate, burner 17 can be operated for a predetermined periodof time to fully combust any residual high-sulfur-containing fuelpresent in the exhaust stream.

Referring to FIGS. 2 and 3, control algorithms of the present disclosurewill now be described. Fuel provided by fuel tanks 13 a and 13 b toengine 12 is controlled through valves 46 a and 46 b, respectively.Valves 46 a and 46 b are in communication with controller 36 and/or ECU34. When it is desired to switch between fuels provided by tanks 13 aand 13 b, controller 36 or ECU 34 can instruct valves 46 a and 46 b toeither open or close. For example, if engine 12 is switching to alower-sulfur-containing fuel from a higher-sulfur-containing fuel,controller 36 or ECU 34 can instruct valve 46 a to close and instructvalve 46 b to open. Before controller 36 or ECU 34 sends instructions tovalves 46 a and 46 b to either open or close, controller 36 or ECU 34can instruct burner 17 to be activated to raise exhaust temperatures toan extent where any un-combusted fuel present in the exhaust stream canbe combusted (e.g., 300 C). In this regard, if a fuel switch is desired,controller 36 or ECU 34 can signal instruct burner 17 to activate, anddelay actuation of valves 46 a and 46 b until burner 17 has operated fora predetermined period of time.

Although it is preferable to activate burner 17 before switching fuels,it should be understood that controller 36 or ECU 34 can activate burner17 simultaneously with the fuel switch, or immediately following thefuel switch, without departing from the scope of the present disclosure.Regardless, during a fuel switch burner 17 should be operated for aduration (e.g., 5-10 minutes) sufficient to combust any unused fuel inthe exhaust stream. In this manner, unnecessary by-products that canplug SCR 20 can be eliminated, or at least substantially minimized.

Although not required by the present disclosure, it should be understoodthat valve 44 can be controlled to open by-pass pipe 40 during the fuelswitch while burner 17 is activated. When the fuel switch is completeand burner 17 is deactivated, valve 44 can close by-pass pipe 40 andallow the exhaust to pass through exhaust after-treatment system 16. Inthis manner, it can be ensured that any un-combustedhigh-sulfur-containing fuel can be prevented from reaching SCR 20.

Moreover, valve 44 can be designed to be synchronized with fuel valves46 a and 46 b. That is, if a signal is sent by controller 36 to fuelvalve 46 b to open such that engine 12 can operate on alower-sulfur-containing fuel, valve 44 can simultaneously receive asignal to close by-pass pipe 40 to allow exhaust to travel throughexhaust after-treatment system 16. Similarly, if a signal is sent bycontroller 36 to open fuel valve 46 a such that engine 12 can operate ona higher-sulfur-containing fuel, valve 44 can simultaneously receive asignal to open by-pass pipe 40 to allow exhaust to be expelled into theatmosphere before passing through after-treatment system 16.

According to another aspect of the present disclosure, a bleedpassageway 50 (FIG. 1) provides communication between by-pass pipe 40and a position located upstream of SCR 20. Bleed passageway 50 cancommunicate a portion of the exhaust travelling through by-pass pipe 40to the position of exhaust passage 14 located upstream SCR 20. Morespecifically, if engine 12 is operating in a manner where exhaust isbeing expelled into the atmosphere through by-pass pipe 40 withoutpassing through after-treatment system 16, SCR 20 may be at atemperature that is substantially less than the exhaust temperature.When it is desired to pass the exhaust through after-treatment system16, valve 44 will be actuated to close by-pass pipe 40 and allow exhaustto enter after-treatment system 16 and the relatively cold SCR. Thetemperature difference between the exhaust gas and SCR 20 may be greaterthan optimal, especially during a fuel switch when burner 17 isactivated to combust unburned fuel and raise the exhaust temperature. Athermal shock event may occur that may cause the substrate (not shown)of SCR 20 to fracture.

To prevent SCR 20 from experiencing large temperature differences thatcan fracture the substrates of SCR 20, bleed passageway 50 can provide apredetermined or metered amount of exhaust flow to SCR 20 to slowlyraise the temperature of SCR 20. A bleed valve 52 can be provided inbleed passageway 50 to allow exhaust gas to enter bleed passageway 50and travel back to exhaust passage 14 at a position located upstreamfrom SCR 20. Bleed valve 52 can be controlled by controller 36 or ECU34, and can be instructed to open if after-treatment system 16 is to beutilized. As exhaust gas is provided to SCR 20 over a period of time,the temperature of SCR 20 will be raised to an extent such that a largetemperature gradient will not be experienced by SCR 20 after valve 44closes by-pass pipe 40.

Bleed valve 52 can also be instructed to open in correspondence with afuel switch instruction being sent to valves 46 a and 46 b. For example,if a fuel switch occurs between a high-sulfur-containing fuel and alower-sulfur-containing fuel, controller 36 or ECU 34 can instruct bleedvalve 52 to open bleed passageway 50 and allow a metered amount ofexhaust gas to be supplied to SCR 20 over the duration of the fuelswitch. The metered amount of exhaust allowed to reach SCR 20 will allowthe temperature of SCR 20 to be raised to an extent such that a largetemperature gradient will not be experienced by SCR 20 after valve 44closes by-pass pipe 40.

Now referring to FIG. 2, a method of eliminating residual fuel in theexhaust that may occur during a fuel switch will be described. In step100, controller 36 will determine whether a fuel switch has beenrequested during operation of engine 12. If a fuel switch has beenrequested, controller 36 or ECU 34 can instruct burner 17 to activate(step 200). Following activation of burner 17, fuel valves 46 a and 46 bcan be actuated to switch between the respective fuels stored in tanks13 a and 13 b (step 300). During the switch between fuels, burner 17 canbe operated for a predetermined period of time (e.g., 10-15 minutes)sufficient to ensure that residual fuel in the exhaust is combusted(step 400). After the predetermined period of time elapses, burner 17may be deactivated (step 500). In this manner, the production of ammoniabisulfate can be substantially reduced to prevent plugging of SCR 20over prolonged periods of operation.

Now referring to FIG. 3, a method of operating exhaust system 16 duringa switch between a lower-sulfur-containing fuel to ahigh-sulfur-containing fuel will be described. In step 600, controller36 or ECU 34 can determine whether a fuel switch has been requested. Ifsuch a switch has been requested, controller 36 or ECU 34 can instructfuel valves 46 a and 46 b to actuate to switch between the differentfuels (step 700), as well as instruct burner 17 to activate (step 800).As discussed above, burner 17 can be activated prior to the actuation ofvalves 46 a and 46 b, simultaneously activated with valves 46 a and 46b, or activated immediately following actuation of valves 46 a and 46 bwithout departing from the scope of the present disclosure.

After actuation of valves 46 a and 46 b and burner 17, controller 36 orECU 34 can instruct by-pass valve 44 to open by-pass pipe 40 to allowexhaust produced during combustion of the high-sulfur-containing fuel toby-pass SCR 20 (step 900). During the switch between fuels, burner 17can be operated for a predetermined period of time sufficient (e.g.,10-15 minutes) to ensure that residual lower-sulfur-containing fuel iscombusted (step 1000). After the predetermined period of time elapses,burner 17 may be deactivated (step 1100). Because exhaust gases producedthrough the combustion of the high-sulfur-containing fuel are expelledto the atmosphere without passing through SCR 20, the production ofammonia bisulfate can be substantially reduced to prevent plugging ofSCR 20 over prolonged periods of operation.

With reference to FIG. 4, a method of pre-heating SCR 20 during a fuelswitch between a high-sulfur-containing fuel to alower-sulfur-containing fuel will be described. In step 1200, controller36 or ECU 34 can determine whether a switch between thehigh-sulfur-containing fuel to the lower-sulfur-containing fuel has beenrequested. If such a switch has been requested, controller 36 or ECU 34can instruct fuel valves 46 a and 46 b to actuate to effect the switchbetween the fuels (step 1300), as well as instruct burner 17 to activate(step 1400). As discussed above, burner 17 can be activated prior to theactuation of valves 46 a and 46 b, simultaneously activated with valves46 a and 46 b, or activated immediately following actuation of valves 46a and 46 b without departing from the scope of the present disclosure.

After actuation of valves 46 a and 46 b and burner 17, controller 36 orECU 34 can instruct bleed valve 52 to open bleed passage 50 to allow asmall amount of exhaust to begin entering SCR 20 (step 1500). Bymonitoring exhaust temperature downstream from SCR 20 (FIG. 1), it canbe determined whether SCR 20 has sufficiently pre-heated (step 1600).Once SCR 20 has sufficiently pre-heated, controller 36 or ECU 34 candetermine whether burner 17 has operated for a predetermined amount oftime to prevent residual high-sulfur-containing fuel from reaching SCR20 (step 1700). If burner 17 has operated for the predetermined amountof time, controller 36 or ECU 34 can instruct bleed valve 52 and by-passvalves 44 to close (step 1800) and instruct burner 17 to deactivate(step 1900).

Although operation of burner 17 has been described above relative to afuel switch, it should be understood that operation of burner 17 canalso assist in abating formation of ammonium bisulfate during periods ofnormal engine operation. SCR 20 is a porous substrate that absorbs ureaexhaust treatment fluid as the urea is dosed into exhaust passage 14 andenters exhaust treatment device 18. Before dosing occurs, SCR 20 can besubstantially devoid of any urea exhaust treatment fluid therein. Asdosing begins, however, SCR 20 will relatively quickly absorb the ureaexhaust treatment fluid and begin the process of reducing NO_(X) tonitrogen gas (N₂) and water (H₂O).

When engine 12 is operating at a relatively high load, a temperature ofthe exhaust can typically be between 350-400 C. Further, duringoperation of engine 12 at this relatively high load, dosing module 22 isgenerally dosing the urea exhaust treatment fluid into the exhauststream to reduce the NO_(X) contained in the exhaust. Due to thesimultaneous presence of urea and sulfur, formation of ammoniumbisulfate can occur. Notwithstanding, when the exhaust temperature isgreater than 350 C, the ammonium bisulfate is generally gaseous andpasses through SCR 20 without substantially adhering thereto.

When the load of the engine 12 is reduced, the temperature of theexhaust can fall to a temperature in a range between 150 to 250 C. Attemperatures less than 180 C, dosing of the urea exhaust treatment fluidcan form deposits in exhaust passage 14. It may not be desirable,therefore, to operate dosing module 22 when the engine load has beenreduced to avoid the formation of the deposits. Although dosing of theurea exhaust treatment fluid may be stopped due to the reduction inexhaust temperature, SCR 20 may still have a relatively high amount ofurea absorbed therein. The formation of ammonium bisulfate, therefore,can still occur. Furthermore, at the reduced exhaust temperatures of 150C to 250 C, the ammonium bisulfate can be in a liquid state that canadhere to SCR 20, which is undesirable.

In the event that engine load is reduced to an extent that the exhausttemperature will also be reduced and urea dosing ceased, burner 17 canbe activated. When burner 17 is activated, the exhaust temperature canbe maintained at a level (e.g., 350 C to 400 C) where ammonium bisulfatecan be in a gaseous state that will continue to pass through SCR 20without adhering thereto. Burner 17 can be operated for a period of time(e.g., 10-20 minutes) sufficient to ensure that any residual ureaabsorbed by SCR 20 will have fully reacted with the exhaust gases. Thefull period of time can be based on measurements taken by NO_(X) sensor32 (e.g., when the NO_(X) content begins to rise, the SCR 20 is mostlikely devoid of urea). Once SCR 20 has completely dried out, burner 17can be deactivated. Because urea is no longer in the exhaust stream,ammonium bisulfate is prevented from forming.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment but, where applicable, are interchangeable and can be used ina selected embodiment, even if not specifically shown or described. Thesame may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. An exhaust system for a dual-fuel engine that is provided with a first fuel and a different second fuel for combustion, comprising: an exhaust passageway; an exhaust treatment component provided in the exhaust passageway, the exhaust treatment component being configured to treat exhaust from combustion of the second fuel and not from combustion of the first fuel; a thermal enhancement device in communication with the exhaust passageway and located upstream from the exhaust treatment component; and a controller configured to activate and deactivate the thermal enhancement device based on switching from the first fuel to the second fuel, wherein the first fuel has a higher sulfur content than the second fuel, wherein the thermal enhancement device increases a temperature of an exhaust to combust a residual amount of the first fuel present in the exhaust passageway during the switch between the first fuel and the second fuel.
 2. The exhaust system of claim 1, wherein the exhaust treatment component is a selective catalytic reduction (SCR) component.
 3. The exhaust system of claim 1, further comprising an exhaust by-pass pipe for by-passing the exhaust treatment component.
 4. The exhaust system of claim 3, further comprising a valve that is operable to open and close the exhaust by-pass pipe.
 5. The exhaust system of claim 3, further comprising a bleed passage in communication between the exhaust passageway and the exhaust by-pass pipe.
 6. The exhaust system of claim 1, wherein the thermal enhancement device is activated before the switch, simultaneously with the switch, or immediately following the switch from the first fuel to the second fuel.
 7. The exhaust system of claim 1, wherein the thermal enhancement device comprises a burner for combusting residual fuel present in the exhaust passageway prior to the residual fuel entering the exhaust treatment component.
 8. The exhaust system of claim 1, wherein the thermal enhancement device is operated for a predetermined amount of time after the supply of the first fuel to the engine is discontinued.
 9. The exhaust system of claim 1, wherein the exhaust generated by engine combustion of the first fuel escapes to atmosphere prior to entering the exhaust treatment component and the exhaust generated by engine combustion of the second fuel passes through the exhaust treatment component, wherein after discontinuing the supply of the first fuel to the ending, the residual amount of first fuel that was not combusted by the engine is combusted by the thermal enhancement device and the exhaust from the thermal enhancement device passes through the exhaust treatment component.
 10. A method of treating exhaust produced by a dual-fuel engine that is operable to switch between a first fuel and a second fuel, comprising operating a thermal enhancement device that raises an exhaust temperature based on switching between the first fuel and the second fuel, further comprising operating the thermal enhancement device to combust a residual amount of the first fuel present in the exhaust emitted by the engine and supplying the exhaust from the thermal enhancement device to an exhaust treatment component.
 11. The method of claim 10, wherein the first fuel has a greater sulfur content than the second fuel.
 12. The method of claim 11, further comprising treating exhaust produced through combustion of the second fuel with the exhaust treatment component.
 13. The method of claim 12, wherein the exhaust treatment component is an SCR.
 14. The method of claim 13, further comprising expelling exhaust produced through combustion of the first fuel into the atmosphere before treating the exhaust produced through combustion of the second fuel with the exhaust treatment component.
 15. The method of claim 11, wherein the thermal enhancement device is operated before switching the fuels, during switching of the fuels, or after switching of the fuels. 